“Isabella was a happy soul, but she would pant and get out of breath, and you could see she needed her operation.”

Ten families are already suing Bristol Royal Hospital for Children over alleged failings which left seven children dead and three with disabilities.

Isabella’s death is one of two new cases that will be the subject of a coroner’s inquest.

By the time the toddler was admitted to hospital in September last year she had a fluctuating blood pressure.

Debbie said: “We were just relieved we had got her to Bristol and she was going to have surgery.

“On the night before surgery I believe they should have checked her blood pressure, but they didn’t because a blood pressure machine wasn’t working.

“When we asked for it to be checked we were told not to worry.”

Debbie claimed: “We asked again in the morning before surgery, but the checks were never made.

“We were naturally anxious about the surgery, but we had been told beforehand that no child at Bristol had ever died from this operation.”

The surgery involved inserting a balloon into Isabella’s heart to widen the blood vessel around the valve to increase blood flow.

It was intended as a temporary measure, with valve replacement surgery to follow “sooner rather than later”.

During the operation Isabella suffered a cardiac arrest from which she never recovered.

A post mortem showed she had also bled internally.

Debbie and husband Daniel, 38, a civil servant, have presented the hospital with a list of 31 questions, none of which have been answered, they say.

Daniel said: “At the moment we can’t grieve properly because we don’t have the answers and we feel the trust isn’t helping us by answering our questions. This was meant to be a very safe operation.”

A Care Quality Commission investigation found serious staffing problems led to a number of deaths between 2008 and 2012, including those of Luke Jenkins from Cardiff and Sean Turner from Wiltshire, whose inquest finishes today.

An inquest into Isabella’s death has been opened but no date has yet been fixed for a full hearing.

A spokesman for Bristol Royal Children’s Hospital said: “In addition to the inquest of Sean Turner and Luke Jenkins, the Avon coroner has asked the trust to provide statements for two further inquests where the children concerned had cardiac surgery at the Bristol Royal Hospital for Children.

“The coroner has a duty to investigate deaths where the cause of death is unknown.

“At Bristol Children’s Hospital we care for some of the sickest children, who come to us for very complex surgical procedures.”